Howdy, howdy!
This is... a bit tricky.
There are different ways: On the one hand, you can look in other stores, e.g. F-Droid or Aurora, or on the other hand, you can work with "Side Loading". The stores work like the Play Store, but you have to install them first - this is where "side loading" comes into play...
Side loading means that (in the case of Android) you organise an apk file of the programme and make it directly available to the box. For example, you can put it on a USB stick or make it available on a network drive. To do this, you must accept "foreign apps" in the settings BEFOREhand: The option on my Fairphone 3+ (with Android 10) is called "Install unknown apps" and I found it under the following path: Settings > Apps & Notifications > Advanced > Special App Access. Of course, you can also just search for it in the settings.
Now for the .apk file. A simple search for "swisscom blue tv app apk download" at ecosia.org gives me these results ->
https://www.ecosia.org/search <- and at androidfreeware.net I seem to have found the current version ->
https://www.androidfreeware.net/download-c... <- First there is a while of information (and a lot of advertising in between) and then a blue button "Download APK 5.3.2". If you click on this button, you will get to a new page and there is another button, this time called "Install APK". Don't worry: Both Firefox and MS Edge only download the file, I haven't tried Chrome.
You then have to feed the downloaded file to the box, but that should be as easy as: Select the file on the data carrier, confirm the question "Do you really want to install a foreign app?
I hope that helps you. As you can see, it's not entirely trivial. You can catch a lot of rubbish with third-party apps - so you should definitely keep your virus protection up to date before downloading the stuff ^^.
And if all the ads annoy you as much as they do me, you can disconnect them! I use Firefox with the extensions "Ghostery", "uBlock Origin" and "Privacy Badger" and have my peace. Sometimes you have to define one or the other exception so that a page continues to work, but the majority runs wonderfully ad-free. At ups.com you can't change the password if cookies are blocked, so I switch off one of the extensions and reload the page. If it doesn't work after that, I try the others step by step and when I have changed my password at the end, I turn all the extensions back on ]:->
So, enough babbling.
-the.